Method of treating metal articles

ABSTRACT

A METHOD OF TOUGHENING HIGH SPEED STEEL AND CARBON STEEL FORMING AND CUTTING TOOLS OR THE LIKE COMPRISING IMMERSING A STEEL OBJECT IN AN AQUEOUS TREATING BATH WHICH INCLUDES A CYANIDE SALT, AN ALKALI METAL HYDROXIDE, A SEQUESTERING AGENT FOR FERRIC IONS AT HIG PH, AND, USING THE ARTICLE AS ONE ELECTRODE PASSING AN ALTERNATING CURRENT THROUGH SAID ARTICLE AND SAID BATH AT A POTENTIAL OF ABOUT 2.5 VOLTS AND A RATE OF ABOUT 1.75 AMPERES PER SQUARE INCH OF SURFACE OF THE ARTICLE FOR A PERIOD OF ABOUT ONE MINUTE OR LESS AT A TEMPERATURE SLIGHTLY ABOVE AMBIENT TEMPERATURE. METALS SO TREATED ALSO DISPLAY OTHER DESCRIBED ADVANTAGEOUS PROPERTIES.

United States Patent 3,709,803 METHOD OF TREATING METAL ARTICLES GrahamL. Gulick, 4559 N. Hamlin, Chicago, Ill. 60625 No Drawing.Continuation-impart of application Ser. No. 833,720, June 16, 1969. Thisapplication Feb. 1, 1972, Ser. No. 222,656

Int. Cl. C23b 1 /00, 3/06 US. Cl. 204140 11 Claims ABSTRACT OF THEDISCLOSURE A method of toughening high speed steels and carbon steelforming and cutting tools or the like comprising immersing a steelobject in an aqueous treating bath which includes a cyanide salt, analkali metal hydroxide, a sequestering agent for ferric ions at high pH,and, using the article as one electrode, passing an alternating currentthrough said article and said bath at a potential of about 2.5 volts anda rate of about 1.75 amperes per square inch of surface of the articlefor a period of about one minute or less at a temperature slightly aboveambient temperature. Metals so treated also display other describedadvantageous properties.

BACKGROUND OF THE INVENTION This application is a continuation-in-partof application Ser. No. 833,720, filed June 16, 1969 and now abandoned.

The present invention is directed generally to methods of metaltreatment, and in particular, to a method of altering thebrittleness-toughness characteristics of metal articles, particularlycarbon and high speed steel cutting and forming tools and the like, toimpart greatly improved toughness thereto.

It is well known that there are a number of thermal methods adapted tobe used in hardening of carbon steel, such as heat treating and thelike. Once an article such as a drill bit, cutting tool or the like hasbeen properly heat treated, it is sufficiently hard for use, but toolstreated in this manner and made from carbon or high speed steel would bemore desirable if they were of increased toughness, that is, if the lifeof such tools could be increased or if the cutting rates or speedsthereof could be increased, or both. Thus, toughness, whichqualitatively may be said to be somewhat the opposite of brittleness, isnot readily measured directly, but is manifested by the ability of thecutting tool or the like to remain sharp, to be able to tolerateincreased speed of metal removal, to resist damage from interruptedcutting operations, and to undergo smaller temperature rise than a lesstough tool under comparable conditions.

The present invention therefore relates to a toughening treatment inwhich a low voltage alternating electrical current is passed through anarticle to be treated in the presence of a suitable electrolyte,resulting in treated metal articles, such as drill bits and the like,having vastly improved toughness.

In view of the fact that high speed cutting tools would be much moredesirable if of increased toughness, and in view of the need for a posttreatment for heat treated articles to provide cutting tools and thelike of increased toughness, it is an object of the invention to providea novel treatment method for toughening ferrous metal articles.

Another object is to provide a method in which an alternating current isimpressed upon an article to be treated, using the article as anelectrode, and subjecting the article to carefully controlled currentfiow while the article is immersed in a chemical treating bath.

Another object is to provide a treating method which includes passing analternating current through a ferrous 3,709,803 Patented Jan. 9, 1973metal article to be treated in the presence of a reducing solution undercontrolled conditions.

Another object of the invention is to provide a method of altering thetoughness/brittleness ratio of a metal article without erosion of themetal, change in the surface finish, or change of dimensions of thetreated article, that is, a method of imparting greater toughness andless brittleness to a metal article having a given hardness.

Still another object of the invention is to provide a method of treatingthe entire body of a metal article while exposing the surface tocontrolled chemical and electrical conditions.

A still further object of the invention is to provide a method oftreating articles under milder conditions than those employed to descaleor remove rust from such articles.

Still another object is to provide a treating method utilizing a voltagesubstantially lower than that used in methods for surface cleaningarticles, which prior art methods serve to bring about an effect in thearticles which is opposite the effect brought about by the method of theinvention.

A still further object is to provide a method of providing tools, suchas drill bits, forming tools, shears, thread forming rollers, and thelike having increased toughness as manifested by longer life, theability to perform a faster cutting operation and characterized by lesstemperature rise than untreated counterparts thereof.

Another object is to provide a method for treating a metal article sothat the article, when used as a transformer core or the like, undergoessmaller temperature rises in use than like untreated articles under thesame conditions.

The present invention achieves these objects and others which areinherent therein by immersing an article to be treated in an aqueoussolution including a cyanide salt, an alkali metal hydroxide, a ferricion chelating agent, and, using the article to be treated as oneelectrode, passing a current through the article and the bath undercarefully controlled conditions of current density and at a potential ofabout 2 to 3 volts. The manner in which the invention is carried outwill become more clear when reference is made to the accompanyingdetailed description of the preferred embodiments set forth by way ofexample, and to the appended claims.

The invention will now be described with reference to several suchpreferred embodiments, which serve to illustrate typical methods ofpracticing the invention.

Example 1 Initially the work to be treated, such as a drill bit or likecutting tool, is chemically cleaned with suitable solvents or otherwiseby known methods to remove grease, oil, or other contaminating films.Care is taken to avoid touching the work with the fingers after cleaningthereof. In the event the articles desired to be treated are rusty orscaly, the rust or scale must be removed therefrom prior to cleaning asset forth above.

An electrolytic solution is then prepared according to the followingformula:

Sodium cyanide-73.5 grams (1.5 moles) Sodium hydroxide40.0 grams (1.0moles) Seqlene ES-50200 ml.

Water to make one liter The Seqlene ES-SO is a 50% aqueous solution ofthe alpha and beta isomers of sodium glucoheptantate, made by PfanstiehlLaboratories, Waukegan, Ill., and is a chelating agent for ferric ironions which is effective at high pH. The bath is placed in a glass lineror otherwise electrically insulated tank adapted to prevent straycurrents, and heated to 40 C. with agitation. Preferably,

tank electrodes are 24 gauge perforated stainless steel (Type 304)sheets, and the surface area thereof is larger than that of the largestsurface area of the work about to be undertaken.

A step down transformer is provided, such as a Variac, and 60 cycle, 110volt alternating current is supplied to the primary thereof, with thesecondary being variable so as to provide from to volts (root meansquare) across the secondary of the transformer. One terminal of thesecondary is connected to the tank electrodes, through a suitable largeammeter, which is preferably and ordinarily an ammeter of the typereading the root mean square current values rather than peak currentvalues (root means square values being those referred to throughout thisdescription). The other terminal of the secondary is connected to aconductive article holder clip for supporting the Work in a completelyimmersed position in the bath. Appropriate calculations and adjustmentsare made to provide an operating potential of 2.5 volts and a currentdensity of 1.75 amperes per square inch of work area, such currentmeasurement being the root mean square current value rather than peakcurrent value. Preferably, the work is disposed between the two tankelectrodes, which are preferably held in a vertical position with theirmajor surfaces in planes parallel to each other. The current is allowedto flow for 40 seconds at the 40 C. temperature, while the 1.75 ampereper square inch current density is maintained. Thereafter, the drill bitor the like is removed and is ready for use without further treatment.

In the case of drill bits treated according to the above set forthexample and used to drill copper-epoxy-glass laminate used in theconstruction of printed electrical circuit boards, for example, theaverage life of such units was about 75% of the life of solid carbidedrill bits costing ten times as much as the high speed steel drill bitstreated for toughness according to the invention. A drill bit preparedin this manner has excellent resistance to shock, and thereby isparticularly advantageous for use where shock conditions are present andtend to damage, even extremely hard drill bits, such as carbide bits. Asan example, an operation involving interrupted cuts creates severeshock, and damages or rapidly destroys such bits or cutting tools which,although very hard, are relatively brittle.

Example 2 A treatment of the tool bit was carried out according to themethod of Example 1, except that the current density was about 2.5amperes per square inch of surface area of the tool. This currentdensity was operative to produce a toughened tool in some cases, but inother cases, appeared to cause slight softening of metal, and on theaverage was not nearly as effective as the 1.75 ampere per square inchcurrent density of Example 1.

Example 3 Treatment of a drill bit was carried out according to themethod of Example 1 except that the current density was held at fromabout 1.0 to 1.25 amperes per square inch. 'In this example, the cyanidesalt was also present in the ratio of 1.5 moles per liter, but was apotassium cyanide salt rather than sodium cyanide, and the alkali metalhydroxide was potassium hydroxide, which was present in -a concentrationof 1.0 moles per liter. On the average, this current density was notsufiicient to produce a significantly improved tool, although in somecases, an improved tool could be produced by this method.

As a result of considering Examples 2 and 3, it was determined that anoptimum current density range is from about 1.25 amperes per square inchor more to about 2.25 amperes per square inch or less, and that 1.75amperes per square inch is preferred. The preferred voltage was 2.5.While voltages of about 2 volts to 3 volts were satisfactory, voltagesof higher values, such as 5 volts or more, produced unsatisfactoryarticles having undesirabl soft and/or brittle surfaces rather thansurfaces of increased toughness.

Example 4 A treating bath was prepared using only about 0.5 mole perliter (24.5 grams per liter) of sodium cyanide, 0.3 mole (12 grams perliter) of sodium hydroxide, and about 50 ml. of the Seqlene ES--50 (50%solution) with a current density of the order of 1.75 ampere per squareinch and a 2.5 volt potential, for a time of between 35 and 45 seconds.Excellent results were also obtained when a tool was treated with thismethod. A tool was also successfully treated using potassium cyanide andpotassium hydroxide in the above set forth molar concentrations.

Example 5 Treatment of a tool bit was again carried out under the sameconditions as those referred to in Example 1, except that approximately4.5 moles per liter (220 grams per liter) of sodium cyanide, about 3moles per liter grams per liter) of sodium hydroxide, and about 250 ml.of Seqlene ES-SO were used. At a current density of 1.75 ampere persquare inch of work, a temperature of 40 C. and a 40 second cycle time,good results were obtained.

Thus, considering Examples 4 and 5, it is apparent that theconcentrations of the cyanide, the alkali metal hydroxide, and theferric ion chelating agent are not particularly critical, although thepreferred concentrations are those set forth in Example 1.

Example 6 A treating operation similar to that described in con: nectionwith Example 1 was carried out, except that the chelating agent used wastriethanolamine. The triethanolamine performs satisfactorily, but ismore costly and not always as effective as the Seqlene ES-SO reagent,and accordingly, is not preferred.

In connection with the methods described in the above examples, andother similar methods, it was also determined that temperatures below 40C. were operable, although temperatures in the 40 C. range tend to givesomewhat increased speed of the treating cycle time than lowertemperatures.

It is also known to those skilled in the art that relatively hightemperatures cause decomposition of the cyanide reagent, andaccordingly, such high temperatures are preferably to be avoided. The 40second cycle time, plus or minus about 4 seconds, is preferred, althoughcutting aid forming tools have been improved by the process of theinvention using time periods falling significantly outside this range.

It is not 'known with certainty why the present invention produces suchremarkable results in increased tool toughness and longevity ofsharpened tool edges as well as tools capable of higher cutting ratesand lower operating tool temperatures, as compared to untreated toolshaving the same speeds and cutting rates; however, it is consideredpossible that the mechanism of hardening a carbon steel includes theadsorption of atomic hydrogen at a number of sites on the surface ofcarbon particles which are located at the grain boundaries, and thatthis adsorption can be controlled by the method of the presentinvention. It is thought that these carbon particles may adsorb hydrogenupon their surface, and under ideal conditions, when the correctquantity of hydrogen is adsorbed on the surface of the particle, it isdiflicult for these metal grains to slip past one another under appliedstress, and the hardness of the metal is greatly increased. However, thenumber of sites at which carbon may adsorb hydrogen are thought to bevaried in number and character. Thus, the carbon particles may contain anumber of sites, con sidered to be primary sites, wherein hydrogen isadsorbed with a release of considerable energy and where such hydrogenis therefore strongly held. Even when all of these sites are filled, itis believed that additional hydrogen may be attracted to additionalsites, which may be considered secondary sites, of relatively moreindefinite number and qualitatively characterized by less energy ofadsorption, and whereto hydrogen is consequently less stronglyattracted.

It is also thought possible that the mobility of atomic hydrogen withinthe metal during ordinary tempering or heat treating is sulficient sothat hydrogen is rapidly attracted to grain boundary carbon duringperiods of proper metal temperature, and that the hydrogen mobility andspeed of adsorption are so great that additional or secondary hydrogenadsorpiton may take place at secondary sites rapidly and in a mannerwhich, undesirably, is difficult to control. Excessive or secondaryadsorbed hydrogen is thought to cause a forcing apart or rift ofadjacent grains, and with inter-granular cohesion of the metal beinggreatly reduced at such points, the metal remains hard but isundesirably brittle. It is believed possible that maximum hardnessoccurs when all so called primary adsorption sites are filled, but thatbrittlenes is significantly increased as secondary adsorption takesplace, whereas when sufficient hydrogen to fill all primary sites onlyis available, maximum toughness is obtained In keeping with the possibleexplanation set forth above, it will be understood that the thermalmobility of hydrogen may be considered, for practical purposes, to havea threshold or minimum critical temperature. In such a case, where themobility is brought about by increased temperature, as a practicalmatter, it is not possible to obtain sufficiently exact hydrogenmobility so that hydrogen will be entirely adsorbed at the primarysites, but not at all at secondary sites, the ideal condition at whichthe metal achieves maximum toughness. Therefore, the method of thepresent invention is one in which this mobility may be controlled byproviding proper electro-chemical conditions, rather than by thermalmeans, and one in which means are provided for the excess hydrogen thusmobilized to escape.

It is thought possible that the present invention provides a lowtemperature method of restoring or increasing the mobility of thisadsorbed atomic hydrogen and by providing a phase boundary at the metalsurface across which excess adsorbed hydrogen may escape, so thatprocessing may take place without increasing the temperature beyondambient temperature, or to the orders of temperature at which thermalmobility of adsorbed or adsorbable hydrogen becomes a significantfactor. It is therefore thought possible that with the general mobilityof all adsorbed hydrogen reduced by reason of reduced ambienttemperature, the release rate of hydrogen adsorbed at secondary sites istherefore subject to accurate control so that the hardness of the parentmetal need not be altered, and yet the toughness thereof can besignificantly increased.

In this connection, it is believed that the passage of alternatingcurrent through a magnetically influenced or magnetically affected metalmay stress the crystal structure of the metal sufficiently to permit thedesired limited mobility of weakly adsorbed hydrogen held on the carbonparticles, particularly in the presence of a suitable electrolyte incontact with the metal surface to provide mobility and escape of excesshydrogen.

It is believed that by using a lower potential, of 2 to 3 volts forexample, than that previously used for the purpose of descaling ferrousmetal articles, desirably mild conditions are used to bring about thecontrolled release of secondary hydrogen. Thus, prior art methods usinghigher voltage might be thought of as analogous to prior art methodsinvolving high temperature, that is, the more severe conditions causeloss of control, and relatively indiscriminate loss of primary andsecondary hydrogen. The present method, however, uses much lower voltageand consequently toughens the metal without causing surfaceernbrittlement' characteristic of prior art descaling methods.

The electrolyte appears to be able to allow limited amount of hydrogento be released from the metal across the phase boundary and the hydrogenmay then be released from the solution in detectable quantities. At anyrate, however, neither the exact behavior of the hydrogen, within themetal or in the solution, nor the existence or the operation of theabove postulated mechanism forms an essential part of the presentinvention, and is not intended to limit the scope thereof.

Referring again to the treated articles and method, the treating bathhas been discovered to have an extremely long service life requiringonly infrequent additions thereto. After long use, crystals of sodiumferrocyanide may be found on the bottom of the tank, possibly resultingfrom the reaction of iron and sodium cyanide in the presence of water toyield sodium ferrocyanide, sodium hydroxide and hydrogen in the presenceof, or under the influence of alternating current.

A principal advantage of the invention is that the treated articles suchas stock high speed steel cutting tools, forming tools and the likemerely taken from the shelf and treated as set forth above will produce2 to 10 times more work pieces before resharpening is necessary thanwill their otherwise identical but untreated counterparts. Resharpeningthese tools by conventional methods does not cause loss of thebeneficial advantages of increased toughness achieved by treatment asset forth above.

Although tools treated according to the present invention give excellentresults when used on any work which is less hard than the tool itself,best results have been obtained on non-ferrous metals and alloys, onnon-magnetic ferrous alloys. and on non-metals among which are thereinforced or filled plastics referred to above.

It is also discovered that increasing the tool working clearances overand above those suggested by cutting tool manufacturers gavesignificantly further improved results, thinner cross sections beingrequired to support the cutting edge of the cutting tool. Experimentalmeasurements have thus far indicated that superior performance resultseven without increased clearance, but that tools treated according tothe above method and with greater clearance display outstanding results.

A still further advantage of the treating method of the invention isthat tools thus treated also display greatly increased resistance toatmospheric corrosion.

In addition to drill bits which were illustrated as being typical toolsupon which the present invention may be practiced with advantage, endmills, ordinary kitchen knives, shears, including those used on paper,those used by barbers, for example, and hand files, for example may beadvantageously treated by this method. In the case of hand files, it hasbeen noted, particularly when using them on softer metals such asaluminum or the like, clogging of the files has been significantlyreduced or eliminated, indicating that the soft metals do not tend tocling to or remain attached to the surface of the cutting metal, andthis results in significantly improved performance by way of increasedproduction rates, and greater convenience in use of the files.

Another area in which the present invention demonstrated unexpectedlyfavorable results was in the field of treating piston rings used forinternal combustion engines. Piston rings treated as set forth above inExample 1 were used in a gasoline engine, and, for reasons which are notunderstood fully, demonstrated extremely long life in use as comparedwith otherwise similar but untreated piston rings.

Other articles which demonstrated unexpected properties when treatedaccording to the method of the invention were the laminations fromelectrical transformer cores. For example, the core of an electricaltransformer was disassembled and the individual laminations treatedaccording to the method of Example 1. When the laminations werereassembled and placed in the same transformer, it was noted that underrelatively large load, a much smaller temperature rise than previouslynoted was experienced. Also, when lighter loads were impressed on thetransformer, no temperature rise was detectable, under the same load,with the treated core, whereas the untreated transformer core underwenta noticeable although unobjectionable temperature rise. Accordingly, atrans former with a treated core had a larger useful load capacity orrating than a transformer with an untreated core.

It will thus be seen that the present invention provides a novel tooltreating method having a number of advantages and characteristicsincluding those herein pointed out and others which are inherent in theinvention.

I claim:

1. A method of treating a ferrous metal article to increase thetoughness thereof comprising immersing said article in an electrolytecomprising an aqueous solution of an alkali metal hydroxide, a solublecyanide salt, a chelating agent for ferric ions at high pH, and passingan alternating electrical current through said article to anotherelectrode immersed in said solution at a potential of from about two toabout three volts and at a current density of from about 1.25 to about2.50 ampers per square inch of article to be treated.

2. A method as defined in claim 1 in which said cyanide salt is selectedfrom the group consisting of sodium cyanide and potassium cyanide.

3. A method as defined in claim 1 in which said alkali metal hydroxideis selected from the class consisting of sodium hydroxide and potassiumhydroxide.

4. A method as defined in claim 1 in which said chelating agent issodium glucoheptanate.

5. A method as defined in claim 1 in which said period during which saidarticle is from about 20 seconds to about one minute.

6. A method as defined in claim 1 in which said cyanide salt is presentin an amount of from about 1.0 to about 2.0 moles per liter and in whichsaid alkali metal hydroxide is present in a quantity of about 0.75 toabout 1.5 moles per liter of solution.

7. A method as defined in claim 6 in which said chelating agent ispresent in an amount of from about 25 to about 250 grams of activeingredient per liter of treating solution.

8. A method as defined in claim 1 in which said treated articlecomprises a cutting or forming tool.

9. A method as defined in claim 1 in which said chelating agent istriethanolamine.

10. A method as defined in claim 1 in which said cyanide salt is presentin an amount of from about 0.45 to about 4.5 moles per liter and inwhich said alkali metal hydroxide is present in an amount of from about0.3 to about 3.0 moles per liter of solution.

11. A method of treating a cutting tool made from a ferrous metalcomprising immersing said article in a bath containing an aqueoussolution of about 1.5 moles per liter of a soluble cyanide salt, about 1mole per liter of an alkali metal hydroxide, about to 200 grams perliter of a ferric ion chelating agent effective at high pH, and, usingsaid article as one electrode, passing a 60 cycle per second alternatingcurrent through said bath at a potential of about 2.5 volts and a rootmean square current density of about 1.75 amperes per square inch ofsurface area of said article, for about 40 seconds.

References Cited UNITED STATES PATENTS 2,057,274 10/1936 Mayhew 204-3,420,760 1/ 1969 Freedman et al 204- 3,207,683 9/1965 Hermann 204-141FOREIGN PATENTS 200,740 1/ 1939 France. 663,217 12/ 1951 Great Britain.1,269,443 5/ 1968 Germany.

JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner U.S.Cl. X.R. 204140.5

